Linear actuators are used to move an object along a straight line, either between two end points or to a defined position. Actuators may be air or hydraulically driven using pressure or they may be driven by electricity. Air or hydraulically driven actuators are cheap and simple in design. They are also easy to control, but they require an air or hydraulic supply which is relatively expensive, especially for small systems.
Electrically driven linear actuators normally incorporate a rotating motor and some kind of transmission means to convert the relatively high-speed rotation of the motor to a low speed linear motion. This transmission means may incorporate a gear box and/or a screw shaft. One common type of linear actuator incorporates a screw shaft with a nut running thereon. The screw shaft extends over the full length of the actuator and sets the operating length of the actuator. Since the nut is held in a non-rotatable state, the nut will be displaced when the screw shaft is rotated by a motor. The nut may incorporate rolling elements, such as balls or rollers, between the screw shaft and the nut. This will allow for a high efficiency actuator with high load transfer and long life. The nut may also engage directly with the screw shaft, i.e. a sliding screw design. In this case, the nut is preferably made of a plastic material.
Another type of linear actuator is known from U.S. Pat. No. 5,491,372. Here, a linear actuator is described, in which a circumferential motor drives an elongated cylinder with an internally threaded bore. An actuator assembly provided with an output shaft and a plurality of transmission rollers is retracted or extended by the rotation of the elongated cylinder. The magnets of the motor are attached to the elongated cylinder and the stator of the motor is attached to the housing, encircling the magnets.
Even though this type of linear actuator may work for some specifications, it is relatively expensive and complicated. There is thus room for improvements.